Microalgae powder alters protein conformation and gluten-starch microstructure to attenuate starch digestibility in...

Current studies on microalgae-enriched foods have mainly focused on baked products, while evidence regarding their effects in steamed bread (a high-moisture, non-baked staple with distinct starch gelatinization behavior) is still limited. In particular, the impacts of microalgae on starch digestibility, microstructure, and flavor characteristics of steamed bread remain unclear. This study investigated the effects of microalgae powder (MP, 1-10%) on the quality, structure, starch digestibility, and flavor profile of Chinese steamed bread. Results indicated that MP significantly improved the nutritional profile, increasing protein from 12.13 g/100 g to 16.25 g/100 g and dietary fiber from 17.61 g/100 g to 20.92 g/100 g at 10% substitution, while reducing total starch from 68.62 g/100 g to 61.69 g/100 g. FTIR revealed molecular rearrangements in protein secondary structures, showing an α-helix to β-sheet transition at low to moderate levels, with β-sheets increasing from 36.0% (control) to 42.1% (4% MP), followed by partial protein unfolding (random coil up to 18.5%) at ≥9% MP. Microstructural observations revealed progressive disruption of the gluten network, along with the retention of ungelatinized starch granules and expanded hydration zones at levels above 7%. These structural changes markedly altered starch digestion, rapidly digestible starch decreased from 51.30 g/100 g to 19.01 g/100 g, while slowly digestible starch and resistant starch increased to 13.28 g/100 g and 31.47 g/100 g, respectively. As a consequence, the hydrolysis index (HI) and estimated glycemic index (eGI) declined from 100.00 to 40.33 and from 94.40 to 42.96 at 10% MP. Texture shifted from firmer and chewier (1-4%) to more elastic but looser structures (≥7%). Low levels (1-3%) produced a cleaner, slightly fruity aroma and achieved the highest sensory acceptance, while higher levels introduced stronger herbal or seaweed notes and darker color. These findings demonstrate that MP offers strong potential for nutritional enhancement of steamed bread, with 1-3% suited for consumer applications and 4-6% for low-GI targeted food development.